This invention relates to ion source devices and in particular to a high brightness electron impact gas phase ion source device for use in connection with a focusing column.
In order to produce a small intense focused ion beam spot, it is preferred to start with the brightest possible ion source. One way to increase the brightness in a gas phase ion source is to increase the plasma density in the source. In an electron impact source, increasing the gas pressure will increase the plasma density. At some gas pressure, depending upon the geometry of the source, the mean free path of both the electrons used to ionize the gas and the ions being extracted will become short enough that the brightness of the source decreases due to collisions in the gas. This discovery is exploited in a novel manner in the invention herein disclosed.
Representative prior art includes existing electron impact source devices manufactured by the assignee of the present invention. While the prior art beam source device operates at a relatively high pressure compared with other known source devices, the closest prior art is a relatively open source and thus has a large area in which the gas pressure is comparatively high. As a consequence of the resultant limit on the mean free path of the ions, there is a practical upper limit on beam intensity.